The present invention concerns new hemiacetals which are formed reversibly if keto compounds are contacted with an aliphatic or cycloaliphatic alcohol. The corresponding hemiacetals differ from the keto compounds from which they are formed with regard to their light absorption in the ultraviolet, in the visible or in the infrared range of wave length, or when the hemiacetal is formed from the keto compound through a reversible reaction with said aliphatic or cycloaliphatic alcohol a luminescence or fluorescence is created or quenched.
The present invention furthermore concerns an optical sensor for the determination of aliphatic or cycloaliphatic alcohols in liquid or gaseous samples, which sensor contains a keto compound which is converted into the new hemiacetal if said sensor is contacted with a corresponding sample which contains said aliphatic or cycloaliphatic alcohol. Through the contact of the the sensor with an alcohol containing sample, its light absorption in the ultraviolet, visible or infrared range of wave length, is changed or a fluorescence or a luminescence is induced or quenched.